The invention relates to coating materials for sensors. In particular, the invention relates to coating materials for chemical sensors.
Sensors generally have varying configurations. Typically, a sensor is provided with a chemically sensitive film (often referred to as a xe2x80x9ccoatingxe2x80x9d) that is applied onto a surface of the sensor, for example onto a surface of a sensor piezoelectric crystal. Interactions of the film with a material to be detected (hereinafter xe2x80x9ctarget materialxe2x80x9d), for example an analyte, can induce a change in at least one of mass and visco-elastic properties of the film. This change can be measured as a shift of the resonance frequency of the sensor""s crystal. Further, the change may be related to the analyte concentration. For detection of analytes of differing natures, film and analyte interactions include, but are not limited to, hydrogen bonding, stacking, acid-base, electrostatic and size/shape recognition.
The sensor""s configuration, materials, and other characteristics vary to define operational characteristics, resonance frequencies, and boundaries for the sensor. For example, differing piezoelectric materials in a sensor substrate operate differently, and thus may define the sensor""s operational boundaries and characteristics. If a sensor comprises a quartz crystal microbalance (QCM) as a sensor substrate, the sensor typically operates by propagating mechanical oscillations generally perpendicularly between parallel faces of a thin, quartz-crystal piezoelectric element. If a sensor comprises a surface acoustic wave (SAW) device as a sensor substrate, mechanical oscillations are generally propagated in substantially up-and-down undulations at a radio frequency (RF) along the surface of a thin, piezoelectric element.
The chemically sensitive film, which can be applied to the sensor, may permit the sensor to more readily detect a target material, analyte, or other compound (hereinafter collectively referred to as xe2x80x9ctarget materialxe2x80x9d), which is not ordinarily sensed by the sensor. The sensitive film often comprises a polymeric material film (hereinafter xe2x80x9cpolymeric filmxe2x80x9d). The polymer film changes the response of the sensor by altering the sensor""s mechanical oscillation frequencies, and thus permits a target material to be detected by the sensor. The sensor""s changing frequencies result from the polymeric film""s interaction with the target material. Accordingly, various target materials can be detected by a sensor when the nature of reaction between the polymeric film and target material is known.
A target material usually a vapor, is dissolved (absorbed) into the film, by a process known in the art as xe2x80x9cpartitioning.xe2x80x9d The absorbing can change properties of the film. The partition coefficient, K, is a thermodynamic parameter that corresponds to an equilibrium distribution of sorbed molecules between the gas phase and polymeric film. The partition coefficient is ratio of a concentration of target material in the polymeric film, CF, to the concentration of the target material outside of the film, CV. The partition coefficient K is determined according to Equation (1)
K=CF/CVxe2x80x83xe2x80x83(Equation 1)
An altered frequency may result from a changed polymeric film mass. An increased film mass lowers a frequency at which the crystal oscillates, including for which it oscillates when exposed to a target material. Thus, the target material perturbs the oscillation of the sensor when the mass of the polymeric film increases, and thus the target material can be detected. The oscillation frequency and mass change of a polymeric film often necessitate that the film be stable, thin, and mechanically rigid. The thin nature of a polymeric film is needed so that the polymeric film""s visco-elastic properties, and any changes in those visco-elastic properties produced by partitioning of the target material thereto, do not adversely influence the oscillations of the sensor and therefore provide inaccurate detection of a target material. Further, the polymeric film should be mechanically rigid so the sensor provided with the polymeric film can have repeated sensing applications.
SAW sensor devices coated with thin polymeric material film (known in the art as xe2x80x9cchemosensorsxe2x80x9d) have been used as micro-sensors for detecting vapors. The sensitivity to a specific vapor (target material) for such a SAW chemosensor depends on the type, physical and chemical properties, and materials of polymeric film. For example, each of a polymer-vapor partition coefficient, rate of absorption, and desorption processes may influence the operational characteristics of a polymeric film. Exemplary polymeric film materials used as on a SAW chemosensor include, but are not limited to, phenyl-methyl-polysiloxanes, poly(epichlorohydrin), poly(isobutylene), poly(ethylene maleate), and poly(ethylenimine). Some of these materials do not provide stable operations in which controlled, accurate, reliable, and repeated detection operations are possible. Although SAW sensors are typically more sensitive than QCM sensors, the relatively low partition coefficients of polymers used in the past normally preclude the use of SAW and QCM sensors for detection of low concentrations of analytes.
Various film materials may be appropriate for detection of some analytes with selected sensors. One known film for sensors, such as piezoelectric and optical sensors, comprises a vapor-sorbing material. The vapor-sorbing material should enhance detecting capabilities of the sensor, however, a vapor-sorbing material may not possess desired mechanical an/or chemical stability characteristics. Other film materials may be incompatible with sensors, provide inaccurate or unreliable results, and generally be unstable and thus unsuitable for use in some environments.
For example, certain film materials may not provide controlled, accurate, reliable, and repeated detection operations. It is known that some sorbing polymer films may exhibit decreased sensing characteristics, including but not limited to, decreased stability and sensitivity during detection operations, when in contact with certain materials and environments. Further, some sorbing polymer films can decrease sensing operational characteristics upon interactions with neat or aqueous solutions of organic solvents. Furthermore, various sorbing polymer films can decrease sensing operational characteristics upon interactions with alkaline solutions. Moreover, some polymer films are chemically unstable, which means that the polymer film may undergo adverse chemical changes after contact with an analyte or target material. Additionally, some polymer films are mechanically unstable, which means that the polymer film may undergo adverse mechanical characteristic changes after contact with an analyte or target material.
Therefore, a need exists for providing a sensor coating material that can retain desirable sensing characteristics if in contact with certain materials and environments. Further, a need exists for a sensor coating material that can provide stable detection operations in which controlled, accurate, reliable, and repeated detection operations are possible if in contact with certain materials and environments.
Accordingly, the invention sets forth a sensor assembly that can detect at least one target material in an environment. The sensor assembly comprises at least one sensor and an amorphous fluoropolymer material coating disposed on a surface of the at least one sensor.
Another aspect of the invention comprises a method for detecting at least one target material. The method comprises: providing at least one sensor; disposing an amorphous fluoropolymer material coating on the at least one sensor to form at least one coated sensor; disposing the at least one coated sensor in an environment that may contain at least one target material, whereby the amorphous fluoropolymer material coating of the coated sensor undergoes changes in response to interactions with the target materials dependent on concentrations of the target material; and relating the changes in the amorphous fluoropolymer material coating to the target material concentration.
A further aspect of the invention provides a monitoring system for at least one of detection, analysis, and evaluation of a target material in an environment. The monitoring system comprises at least one sensor; an amorphous fluoropolymer material coating disposed on a surface of the at least one sensor; and a device that analyzes and evaluates a change in the amorphous fluoropolymer material coating due to interactions with the target material.
In another aspect of the invention, a monitoring system is provided for at least one of detection, analysis, and evaluation of a target material in an environment. The monitoring system comprises means for sensing a target material in an environment; an amorphous fluoropolymer material coating disposed on a surface of the means for sensing; and means for analyzing and evaluating a change in the amorphous fluoropolymer material coating due to interactions with the target material.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.